This invention relates to a communications system and, more particularly, to a communications system in which a plurality of transmission devices provided on the subscriber side and a single transmission device provided on the side of a switch are connected by high-speed digital transmission lines and subscriber terminals are connected to the transmission devices on the subscriber side, whereby the subscriber terminals are accommodated by the switch.
In currently existing subscriber-based optical transmission systems, the configuration is such that a plurality of COT transmission devices (COT is the abbreviation of xe2x80x9cCentral Office Terminalxe2x80x9d) are provided on the side of a switch, a plurality of RT transmission devices (RT is the abbreviation of xe2x80x9cRemote Terminalxe2x80x9d) are provided on the subscriber side and each COT transmission device and its corresponding RT transmission device are connected by a single high-speed digital transmission line (e.g., a high-speed optical transmission line). The RT transmission devices are deployed at remote locations, such as in office buildings, towns and villages.
FIG. 10 is a block diagram illustrating the configuration of a subscriber-based optical transmission system according to the prior art. The system includes an exchange 1 having a switch 2 and COT transmission devices 3A-3N, ordinary household telephones or public telephones 4A-4N provided at remote locations, RT transmission devices 5A-5N provided at remote locations and optical transmission lines 6A-6N for connecting the COT transmission devices 6 and corresponding RT transmission devices.
The RT transmission devices 5A-5N respectively include subscriber circuits 71-7n connected to corresponding telephones, CODECs (coder/decoders) 81-8n for converting analog signals, which enter from the subscriber circuits, to digital signals and converting digital signals, which enter from the side of the transmission lines, to analog signals, channel units 91-9n for performing a unipolar/bipolar conversion and for inserting and extracting various control data, a multiplexer/demultiplexer 10 for multiplexing digital signals, which enter from the CODECs via the channel units, and sending the multiplexed signals to the side of the optical transmission lines, and for demultiplexing multiplexed data, which enters from the side of the optical transmission lines, and outputting the demultiplexed signals to prescribed channels units, and optical terminal equipment 11 for converting electric signals to optical signals and sending the optical signals to the optical transmission lines 6A-6N, and for converting optical signals, which enter from the optical transmission lines, to electric signals and outputting the electric signals.
The COT transmission devices 3A-3N respectively include channel units 121-12n for performing a unipolar/bipolar conversion and for inserting and extracting various control data, a multiplexer/demultiplexer 13 for multiplexing digital signals, which enter from the switch 2 via the channel units, and sending the multiplexed signals to the side of the optical transmission lines, and for demultiplexing multiplexed data, which enters from the side of the optical transmission lines, and outputting the demultiplexed signals to the switch, and optical terminal equipment 14 for converting electric signals, which enter from the multiplexer/demultiplexer, to optical signals and sending the optical signals to the optical transmission lines 6A-6N, and for converting optical signals, which enter from the optical transmission lines 6A-6N, to electric signals and outputting the electric signals.
FIG. 11 is a block diagram illustrating the detailed construction of an RT transmission device.
Remote telephones 4A with which the RT transmission device 5A is provided are divided into n groups of m telephones each. The m telephones 411-41m of a first group are connected to subscriber circuits 711-71m of a first group and the subscriber circuits 711-71m are in turn connected to CODECs 811-81m. The CODECs 811-81m convert voice signals, which enter from the telephones 411-41m, to 64-Kbps digital signals and input the digital signals to the multiplexer/demultiplexer 10 via channel devices 911-91m. The multiplexer/demultiplexers 10 of the respective groups concentrate and time-division multiplex (mxe2x89xa632) 64-Kbps digital signals of a maximum of 32 channels, which have entered via the m channel units 911-91m, and output the multiplexed signals on signal lines L1-Ln as 2-Mbps digital signals. A multiplexer/demultiplexer 10xe2x80x2 further multiplexes the 2-Mbps time-division multiplexed data, which enters from the multiplexer/demultiplexers 10 of the 1st-nth groups via the signal lines L1-Ln, to 34-Mbps or 150-Mbps data (assumed here to be 34-Mbps data) and outputs the data to the optical transmission line 6A from the optical transmission unit 11.
Further, 34-Mbps time-division multiplexed data, which has entered from the switch 2 via the optical transmission line 6A and optical transmission unit 11, is demultiplexed to 2-Mbps time-division multiplexed data by the multiplexer/demultiplexer 10xe2x80x2, and this data enters the multiplexer/demultiplexers 10 of the 1st-nth groups. The multiplexer/demultiplexer 10 of each group converts the 2-Mbps time-division multiplexed data to 64-Kbps digital data of a maximum of 32 channels and inputs the digital signals to prescribed CODECs via the channel devices. Each CODEC to which a digital signal has been input converts the digital signal to an analog signal and inputs the analog signal to a telephone.
FIG. 12 is a block diagram showing the construction of the exchange. The COT transmission devices 3A-3N are provided to correspond to the remote RT transmission devices 5A-5N and are connected to the RT transmission devices 5A-5N via the optical transmission lines 6A-6N. Further, the COT transmission devices 3A-3N are connected to the switch 2 via n-number of the 2-Mbps signal lines L1-Ln. The 34-Mbps time-division multiplexed data sent from the RT transmission devices 5A-5N enter the COT transmission devices 3A-3N via the optical transmission lines 6A-6N. The optical transmission unit 14 in each of the COT transmission devices 3A-3N converts the optical signal to an electric signal and inputs the electric signal to the multiplexer/demultiplexer 13. The latter demultiplexes the 34-Mbps time-division multiplexed data to 2-Mbps digital signals and inputs the demultiplexed signals to the switch 2 via the channel units 121-12n and signal lines L1-Ln. The multiplexer/demultiplexers 13 of the respective COT transmission devices 3A-3N multiplex the 2-Mbps digital signals, which have entered from the switch 2 via the signal lines L1-Ln and channel units 121-12n, to 34-Mbps data and send the data to the optical transmission lines 6A-6N from the optical transmission units 14.
In accordance with this subscriber-based optical transmission system, (1) the RT transmission devices 5A-5N correspond to the COT transmission devices 3A-3N within the exchange 1, and (2) the 2-Mbps time-division multiplexed data on the signal lines L1-Ln (FIG. 11) within the RT transmission device 5A matches the 2-Mbps time-division multiplexed data on the signal lines L1-Ln (FIG. 12) within the exchange 1. Similarly, the 2-Mbps time-division multiplexed data on the signal lines L1-Ln within the RT transmission devices 5B-5N matches the 2-Mbps time-division multiplexed data on the signal lines L1-Ln connected to the COT transmission devices 3B-3N within the exchange 1.
As a result, regardless of the fact that the COT transmission devices 3A-3N on the switch side and the RT transmission device 5A-5N at the remote areas are connected by the single optical transmission lines 6A-6N, the arrangement is functionally equivalent to connecting the signal lines L1-Ln of the 2-Mbps time-division multiplexed data of each group of each RT transmission device directly to the switch 2. This is advantageous in that the cost of laying optical cable can be reduced.
Thus, in the conventional subscriber-based optical transmission system, the COT transmission devices are provided within the exchange so as to correspond to the RT transmission devices, and each COT transmission device requires, in addition to optical terminal equipment and a multiplexer/demultiplexer, channel units the number (n) of which is the same as the number of groups within the corresponding RT transmission device. When the amount of traffic of each subscriber telephone is large, the number of subscriber telephones constituting a group decreases and the number n of groups increases so that the number of channel units constituting the COT transmission device increases accordingly.
The periods of time subscriber telephones are used in remote areas such as business districts and residential areas differ. That is, telephone use at night is greater in residential areas, while telephone use during the day is greater in business districts. Consequently, when RT transmission devices are provided in business districts and residential areas and connected to the same exchange, the total amount of telephone use does not vary that much by period of time. Nevertheless, the conventional arrangement is such that each COT transmission device is required to be provided with hardware (a number of channel units) that anticipates the maximum traffic in each remote area.
The problem that arises is an increase in the quantity, size and cost of the exchange hardware.
Accordingly, an object of the present invention is to reduce the number of channel units in an exchange, thereby making it possible to reduce the size and lower the cost of the exchange.
Another object of the present invention is to reduce the number of channel units by sharing the channel units provided in an exchange with each of the RT transmission device connected to the exchange.
A further object of the present invention is to statistically measure the temporal transition of traffic in advance and share channels units, which are provided in an exchange, with the RT transmission devices based upon the previously measured traffic, thereby reducing the number of channel units.
The maximum amount of traffic of individual transmission devices on the subscriber side does not occur at the same timing. That is, traffic peaks at different timings. The present invention takes note of this fact and decides the hardware of the transmission device on the side of the switch in such a manner that congestion will not occur, even when traffic peaks. This makes it possible to reduce the amount of hardware (e.g., the number of channel units) necessary.
More specifically, a transmission device on the subscriber side converts analog signals from subscriber terminals to digital signals to multiplex the digital signals group by group, time-division multiplexes the multiplexed data from each group and sends the data to the transmission device on the switch side. Further, the transmission device on the subscriber side senses the traffic state (e.g., on-hook/off-hook state) of each subscriber telephone and sends the sensed state to the transmission device on the switch side. The transmission device on the switch side (1) demultiplexes high-speed time-division multiplexed data, which is sent from each transmission device on the subscriber side, to time-division multiplexed data on a per-group basis; (2) identifies, based upon traffic state information, a group in which a subscriber terminal in the off-hook state (communicating state) resides; (3) connects only a time-division multiplexed data sending line corresponding to this group to an unused channel unit; and (4) inputs the time-division multiplexed data to the switch via this channel unit. If this arrangement is adopted, the transmission device on the switch side does not require that time-division multiplexed data sending lines of a group in which all subscriber terminals are in the on-hook state be connected to a channel unit. As a result, the number of channel units provided in an exchange can be reduced, thereby making it possible to reduce the size and lower the cost of the exchange.
Further, the transmission device on the switch side statistically obtains the temporal transition of traffic through each transmission device on the subscriber side in advance, decides the number of channel units allocated to the transmission devices on the subscriber side at each point in time based upon the amount of traffic undergoing a transition with time, and, on the basis of the number of channel units decided, performs control to connect channel units and lines which send time-division multiplexed data, on a per-group basis, sent from each of the transmission devices on the subscriber side. If this arrangement is adopted, the number of channel units provided in an exchange can be reduced, thereby making it possible to reduce the size and lower the cost of the exchange.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.